Thats it!!
At the left hand, buttons are organized by couple of octaves according to the order of fiths.. It is complicated to explain with words but with the vidéo it should be easier to understand...

Where C4 is middle C (I could be confused about the specific octaves).

At first sight It seems to be designed to play oom-pah in keys with a couple of sharps or flats, as the fundamental, third and fifth are nearby. I wonder who first came up with this, Thomas Restoin perhaps (Tona?)

Bravo ocd ! You are right!
You also guess well that I wanted to play rythmic and harmonic accompaniement with the left hand. Not only counterpoint that’s why I did not choose a mirror keyboard of the right hand for example. It seems tricky but it works for me and the music I want to play!

I met Malo a few years ago. We talked about different system of keyboard but he had more reflexion about it and it was difficult for me to follow all his explanations! Very nice and enthusiast guy. He plays his own system accordion and music from Brittany.